Pending U.S. patent application Ser. No. 08/928,996, entitled "Catalytic Converter", filed on Sep. 12, 1997, and assigned to the assignee of this invention discloses a converter having a housing with a ceramic honeycomb catalyst coated substrate located within the cavity of the housing. The substrate is characterized in that its inlet face and its outlet face are each formed with a concave depression, and the central portion of the substrate is wrapped with a mat of intumescent material which expands radially when heated by the exhaust gas of an internal combustion engine. The mat extends along the length of the substrate up to the deepest point of the depression in order to protect the fragile end portions of the substrate and prevent them from fracturing when the converter reaches operating temperature. In addition, the end portions of the substrate are configured so as have the peripheral ends thereof in close proximity to end members that are sealingly attached to the opposed ends of the housing. As a result, the concave depressions in the substrate not only form a gas inlet chamber and a gas outlet chamber but, in addition, the opposed faces of the substrate are intended to shield the supporting mat from the hot gases of the internal combustion engine.
Although the concave facial design of the substrate does provide a shielding effect for the support mat to a certain extent, it is clear that a perfect seal cannot be obtained between the ceramic end portions of the substrate and its associated metallic end member. As a consequence, the ends of the main support mat can be subjected to exhaust gases at a temperature which can be in excess of 750 degrees centigrade. The exhaust gases at this high temperature can damage the ends of the mat and eliminate the mica's ability to expand. The ceramic fibers and mica in the mat then become relatively loose and erode away and cause the mat to deteriorate and lose its ability to serve as an insulator and support for the substrate within the housing.